The term broadband refers generically to any wideband transmission medium. Broadband coaxial communications systems, as the name implies, transport multiple information signals using coaxial cable. In use for video for over thirty years, broadband coax has only recently found widespread use for the carriage of voice and data communications. The large bandwidth available allows large numbers of services to be carried on a single cable.
An understanding of broadband is most easily obtained by contrasting it with baseband communications. In a baseband system, the information signal (i.e. voice, data or video) may be connected directly to the cable. In this manner the service has access to the full transmission channel, but only one service may utilize the cable at any given time. Broadband utilizes a technique similar to that employed by all radio and television broadcasters, i.e. RF (radio-frequency) modulation. The baseband information is first modulated onto an RF carrier before being applied to the cable. By using many different carriers, multiple signals can be handled on the same transmission path.
The transportation medium in a broadband system is coaxial cable. A typical broadband coax system supports carrier frequencies from 5 MHz to 450 MHz. Many services may use the same cable simultaneously by operating each on a different carrier.
As with any transmission medium, the RF signals travelling along the cable are attenuated, or lose their power. Unless the system is relatively short, RF amplifiers are necessary to boost the signals back to their original levels. Broadband amplifiers used in LANs will oftentimes be referred to as bidirectional amplifiers. To provide two-way communications on a broadband network, the available bandwidth of the cable is split in two. The two resulting bandwidths are referred to as the outbound and inbound bandwidths. These two bandwidths carry the information signals in opposite directions. A typical midsplit broadband amplifier, for instance, passes 1-120 MHz in one direction (inbound) and 150-450 MHz in the other (outbound). This provides full-duplex communications on a single cable.
Broadband communications technology similar to that developed for the Cable Television (CATV) industry is often used in local area networks.
Conventional broadband coaxial cable local area networks are generally arranged in what is known as a "tree-and-branch" configuration utilizing equipment and technologies developed for CATV systems. Such a network has a headend, which illustratively has a central computer facility. A signal to be distributed to locations throughout the facility leaves the central computer facility on a "trunk" cable. The "trunk" cable characteristically has no user devices connected, and is intended to transport the signals carried on the system to a general area where "users" are to be located. Most systems require "trunk amplifiers to amplify the RF signals carried on the "trunk" cable at various locations. In addition, the trunk cable can be split over two or more paths, "branching" out to several general areas where users are located.
User devices are connected to the system via a "tapped feeder" cable. This tapped feeder portion of the network emanates from "bridging" amplifiers that isolate the "trunk" from the "tapped feeder" portion of the system. In some cases "terminating trunk" or "terminating bridger" amplifiers are used to delineate the point at which a "tapped feeder" portion of the system begins and the "trunk" portion of the system ends. Tapped feeder cables are generally arranged as a serial connection of several relatively short (compared to the length of trunk cable) lengths of coaxial cable and tap units. In addition, the tapped feeder cable can be split into two or more cables, providing several portions of serially connected taps and cables. Each user device such as a computer terminal is connected to a tap by means of a drop cable of limited length.
Problems with such conventional CATV equipment and architecture in broadband local area networks include a high degree of complexity in network design and component specification, inflexibility for growth and rearrangement which causes network costs to be artificially high, relatively reduced reliability and fault correction time due to the relatively high number of serially connected devices, and relatively higher installation and maintenance costs associated with the nature and complexity of conventional CATV hardware used to implement these networks. Each of these problems will be discussed briefly below.
The high degree of complexity in network design and component selection is due to two factors. First, the tapped feeder portion of the network requires that path loss calculations be performed for each tap unit in both the inbound and outbound directions at two or more frequencies. This path loss calculation is typically done by specialists acting as consultants who in turn utilize specialized computer programs developed and marketed specifically for performing broadband LAN system designs. Secondly, the bi-directional amplifiers used in LANs based upon CATV technology require specification of several accessories necessary to perform independent "balancing" of the gain versus frequency characteristics for both outbound and inbound amplifiers. This requires that proper plug-in attenuators and plug-in cable equalizers be specified as well as the knowledge of the limitations of continuously variable attenuators and equalizers which are provided with each bidirectional amplifier. In addition, path loss calculations must be performed for at least two frequencies in each of the inbound and outbound signal paths for the trunk portion of the system.
The inflexibility of conventional broadband LANs to accommodate growth and rearrangement is primarily due to the nature of the tapped feeder portions of the system. Each tapped feeder portion of the system comprises several serially connected short lengths of coaxial cable and tap units. The decibel value of attenuation of the tap units is selected to properly balance the path loss in both inbound and outbound directions at that particular location. This path loss is dependent upon devices connected between the tap unit and the headed. If, for instance, a change in the system was required to add an additional tap unit at a location that has other tap units located downstream in the outbound direction, it would be not only necessary to perform calculations for selection of the tap to be added to accommodate the additional outlets, but recalculation of path loss for each and every tap unit located beyond the expansion point would also be required. If the calculations resulted in performance specifications which did not meet system requirements, then these tap units might have to be respecified and reinstalled. A conventional solution to this problem is to initially build the network with excess capacity. Thus, extra taps are provided corresponding to extra user positions. This is generally wasteful since the network must be built to a capacity, the need for which is unknown. Typical tap outlet utilization in conventional broadband LANs is in the neighborhood of 10 to 15%. This over-installation results in higher system installation costs.
The relatively reduced reliability and higher fault correction time of conventional broadband LANs is due to the architecture of the system that employs a multiple of serially connected devices. Failure of a component can cause system failure for all devices connected to the system beyond the point of failure. Minimizing the number of serially connected devices will reduce system fault correction time and improve reliability.
Higher installation and maintenance costs are associated with utilizing CATV hardware and architecture in conventional broadband LANs. The CATV hardware is typically packaged in a manner which allows use in either one-way or two-way systems. CATV hardware universally employs separate modules for outbound and inbound amplifiers with separate adjustment of both outbound and inbound amplifiers requiring selection of up to four adjustable components in each outbound and inbound amplifier, these adjustments being selection of a plug-in pad, variable attenuator, plug-in cable equalizer, and variable cable equalizer. Installation of this complex amplification hardware requires trained technicians with extensive experience in CATV technology. In addition, higher maintenance costs are associated with both the complexity of the hardware and the higher fault correction time mentioned previously.
Recent performance standards for broadband LAN systems including IEEE 802.7 Broadband Local Area Network Recommended Practices and General Motors Manufacturing Automation Protocol (MAP) Broadband Media Specifications have been published. These performance standards specify important network performance criteria which might be implemented by a broadband LAN.
Accordingly, it is the object of the invention to provide a LAN which is easier to design, install, maintain, rearrange and expand than prior art LANs as well as being of lower cost than prior art LANs. It is also an object of the invention to provide a LAN which meets or exceeds performance standards required by IEEE 802.7, MAP and other specifications.